1. Field of the Invention
The present invention relates generally to a liquid-gas pumping apparatus for pumping cryogenic liquids from a storage tank, which apparatus includes a vertical housing extending into the tank which functions as a pumping line, an immersion pump and motor unit which is slidably insertable into the housing, and valve means disposed at the lower end of the housing which opens when the immmersion pump and motor are lowered into the housing and closes when the pump and motor are raised in the housing.
2. Description of the Prior Art
Immersion pump and motor units in which the pump impeller and the rotor of an electric motor are mounted on a single shaft are known in the art and are generally used for pumping liquids. Such immersion pump and motor units are installed at the vertically lowest point of a tank or well so that the tank or well can be emptied to as great an extent as possible. It is also known in the art to use such pumps to empty liquid-gas tanks. In such applications, however, it must be possible to empty the tank in the event the pump fails. This can be accomplished, for example, by forcing the liquid out of the tank. There are limits to what can be achieved by this method, however, since liquid-gas tanks cannot be subjected to arbitrarily high stress pressures, and utilization of this method is not without danger.
In one such apparatus described in U.S. Pat. No. 3,369,715, a movable immersion pump and motor unit is introduced into a liquid-gas tank through a lock and well-like housing which extends downwardly to the bottom of the tank. The pump and motor unit is lowered to the bottom of the tank to empty the tank, and the electrical connection to the immersion pump and motor unit is effected by means of electrical cables which are lowered into the tank with the immersion pump and motor unit. A single foot-valve is located at the bottom of the liquid-gas tank which is engaged by the movable immersion pump and motor unit and opens toward the bottom of the tank. In this design, the foot valve is opened by the weight of the pump and motor unit only if the force required to open the foot valve is less than the weight of the pump and motor unit.
The opening force produced by the level to which the tank is filled with fluid and the density of the fluid to be pumped are usually greater than the weight of the pump and motor unit so that it is necessary to use a linkage with a spindle to assist with the operation of the foot valve. This linkage extends through the top of the tank to either the foot valve on the bottom of the tank or to the pump and motor unit. In order to prevent buckling, the linkage is guided through supports at the linkage or in the housing. In large tanks, the linkage is subdivided because of the great height of the tank and comprises subsections during lowering. As a result, the immersion pump and motor unit can be only lowered incrementally which causes a corresponding loss of time. The vapors and gases escaping from the tank during assembly of the linkage must also be drawn off to prevent the formation of a dangerous situation. A stuffing gland seal is used to seal the linkage to the tank top in a pressure-proof manner at the point where the linkage extends through the tank top. A hand wheel for operating the linkage is provided to permit axial movement of the linkage by means of a threaded spindle.
The parts of the immersion pump and motor unit which are subjected to the most danger are the electric supply lines and the stuffing gland seals for the operating linkage of the foot valve. The insulation of the electrical supply cables easily becomes brittle, and therefore useless, particularly when the pump and motor unit is removed from a tank filled with liquid gas or from a tank from which the liquid gas has been discharged and in the interior of which a temperature of about 100 K can prevail. A similar weakness is the stuffing gland seal since it is likewise exposed to the low temperature of the liquid gas.